1. Field of the Invention
The present invention relates to a comparator circuit suitable for use in, for example, a semiconductor integrated circuit testing apparatus for testing and measuring various types of semiconductor integrated circuits.
2. Description of the Related Art
In a semiconductor integrated circuit testing apparatus (hereinafter referred to as IC testing apparatus) for testing and measuring a semiconductor integrated circuit (hereinafter referred to as IC), by way of example, if an IC to be tested (IC under test) is a digital type IC directed to a digital signal processing, whether or not the voltage of a response signal outputted from this IC under test has a prescribed voltage value of logical H (logical high level) or a prescribed voltage value of logical L (logical low level) is determined by a comparator circuit. Specifically, the comparator circuit determines whether the voltage of logical H outputted from the IC under test has a voltage value equal to or higher than the prescribed voltage value or not, or the comparator circuit determines whether the voltage of logical L outputted from the IC under test has a voltage value equal to or lower than the prescribed voltage value or not. When the determination result is "pass" (the response signal has a prescribed voltage value), the comparator circuit outputs a signal of logical H or logical L indicating "the pass result", and this logical signal is sent to a logical comparator circuit for inspecting whether the response of the IC under test is correct or not.
FIG. 4 shows an example of the comparator circuit of this type used in a conventional IC testing apparatus. Although only one voltage comparator VCP used in the comparator circuit is shown in FIG. 4, in a practically used IC testing apparatus both the voltage of logical H and the voltage of logical L are compared and determined in the comparator circuit as to whether they have the prescribed voltages, respectively, and hence two voltage comparators are used in the comparator circuit. Here, since the comparisons of the two voltage values are not the requirements of the present invention, only one voltage comparator VCP for performing a voltage comparison for logical H is illustrated for clarity of the explanation, and the operation thereof will be described.
The voltage comparator VCP has, in this example, two input terminals one of which is a non-inverting input terminal and the other of which is an inverting input terminal, and an output terminal. The inverting input terminal is connected to a comparison (reference) voltage source 16 for outputting a prescribed comparison (reference) voltage V.sub.REF.
In a readout mode in which a test pattern signal already written in an IC under test 10 is read out therefrom, as the IC under test is viewed from the outside, it can be considered to have a voltage output source 11 therein. Assuming that the voltage output source 11 outputs a voltage V.sub.O, this voltage V.sub.O is supplied to one input terminal (in this example, the non-inverting input terminal) of the voltage comparator VCP via a signal line 12 from one terminal to which the voltage output source 11 is connected. Herein, the voltage V.sub.O outputted from the voltage output source 11 will be referred to as an input voltage to the voltage comparator VCP. The voltage comparator VCP functions, in this example, as a subtracter for subtracting the comparison voltage V.sub.REF from an input voltage applied to the non-inverting input terminal thereof and outputting a voltage of logical H or a voltage of logical L.
The signal line 12 for electrically connecting between the IC under test 10 and the one input terminal of the voltage comparator VCP has a resistance component. A total resistance component that exists in the input circuit for the voltage comparator VCP including this resistance component in the signal line 12 and a resistance component in a switch (not shown) for disconnecting the voltage comparator VCP from the IC under test 10 is denoted by a reference character 13, and its resistance value is represented by RON. In a write mode in which a test pattern signal is written in the IC under test 10, a driver (not shown) is connected to the IC under test 10, and the voltage comparator VCP is disconnected from the IC under test 10. The above switch is one for disconnecting the voltage comparator VCP from the IC under test 10 in the write mode, and a relay or a semiconductor switch element is usually used as such switch.
In order to take an impedance matching on the signal line 12, a terminating resistor (terminator) 14 is connected to the signal line 12, and further a terminator voltage source 15 for the terminating resistor 14 is provided. For example, in the case that the signal line 12 has its characteristic impedance of 50 .OMEGA., the resistance value of the terminating resistor 14 is also set to 50 .OMEGA., theby to terminate the terminal end of the signal line 12 with 50 .OMEGA. to maintain the signal line in the state that no reflection occurs.
Assuming that an output voltage of the terminator voltage source 15 is VTT, the resistance value of the terminating resistor 14 is RT and a connection point or junction between the terminating resistor 14 and the resistance component 13 that exists in the input circuit for the voltage comparator VCP is A, an electric potential at the connection point A is applied to the one input terminal of the voltage comparator VCP since the connection point A is connected to the one input terminal of the voltage comparator VCP. The electric potential of the connection point A is divided in voltage by a voltage dividing resistance circuit composed of the resistance value RON of the resistance component 13 and the resistance value RT of the terminating resistor 14. Accordingly, assuming that the divided voltage is V.sub.O ', this divided voltage V.sub.O ' is supplied to the one input terminal of the voltage comparator VCP.
The output voltage VTT of the terminator voltage source 15 is set to just the middle voltage between a voltage VH of logical H and a voltage VL of logical L outputted from the voltage output source 11 of the IC under test 10 (1/2 of the voltage of (VH+VL)). Due to such terminator voltage VTT, the electric potential at the connection point A is maintained at the voltage VTT when the output terminal of the IC under test 10 becomes high impedance mode. As a result, even if the IC under test 10 outputs a voltage of logical H or a voltage of logical L after that, a change in electric potential at the junction A is limited to half (1/2) of the voltage (VH-VL). Accordingly, the provision of the terminator voltage source 15 results in that the electric potential at the junction A can reach the target voltage value at high speed. For this reason, the terminator voltage source 15 is usually connected to the terminating resistor 14.
When the voltage V.sub.O ' at the connection point A is inputted to the one input terminal (in this example, non-inverting input terminal) of the voltage comparator VCP, and the comparison voltage V.sub.REF outputted from the comparison voltage source 16 is inputted to the other input terminal (in this example, inverting input terminal) of the voltage comparator VCP, the voltage V.sub.O ' being a divided voltage of the input voltage V.sub.O outputted from the IC under test 10, the voltage comparator VCP outputs a voltage of logical H as shown in FIG. 6B if the voltage V.sub.O ' of the connection point A is higher than the comparison voltage V.sub.REF (V.sub.O '&gt;V.sub.REF), and outputs a voltage of logical L if the voltage V.sub.O ' of the connection point A is lower than the comparison voltage V.sub.REF (V.sub.O '&lt;V.sub.REF).
As mentioned above, in the comparator circuit in the IC testing apparatus, since the IC under test 10 is connected to the one input terminal of the voltage comparator VCP by the signal line 12, there is a drawback that the operation of voltage comparison is influenced by the resistance component 13 that exists in the input circuit for the voltage comparator VCP.
Specifically explaining, assumed that the voltage outputted from the voltage output source 11 of the IC under test 10 is V.sub.O, the resistance value of the resistance component 13 is RON, the resistance value of the terminating resistor 14 is RT, and the output voltage of the terminator voltage source 15 is VTT as mentioned above, the voltage V.sub.O ' of the connection point A is expressed by the following equation. ##EQU1##
In the above equation, if RON is zero (RON=0), V.sub.O ' is equal to V.sub.O (V.sub.O '=V.sub.O), and hence no error is produced. However, if a semiconductor switch (for example, photo MOS relay) is used as a switch for disconnecting the voltage comparator VCP from the signal line 12, RON has a resistance value of approximately 1.OMEGA. or so. Accordingly, if RT is assumed to be 50 .OMEGA., an error of approximately 2% (about 1/50) will occur in V.sub.O '. If V.sub.O ' should have been changed to a voltage lower than the comparison voltage V.sub.REF as shown in FIG. 6A due to the occurrence of such error, the voltage comparator VCP outputs a voltage of logical L as shown in FIG. 6C, notwithstanding that a voltage of logical H has been outputted from the IC under test 10. As a result, an erroneous comparison result is outputted from the voltage comparator VCP. Therefore, there is a problem that in the conventional comparator circuit even such a very small error as approximately 2% or so cannot be disregarded.